Internal combustion engine technology continues to advance. Improvements in reliability, emissions quality and fuel efficiency are revealed on an almost daily basis. Over the last several decades, particular attention has been paid to technologies capable of reducing the levels of certain emissions in exhaust from internal combustion engines. One class of emissions compounds of special interest is known in the art as “NOx”, and includes various nitrogen-oxygen compounds. Various schemes have been proposed over the years for operating internal combustion engines such that the relative quantities of NOx in the engine exhaust are reduced. One approach showing much promise involves running the engine relatively lean, such that the amount of fuel in the mixture combusted in the cylinder is less than a stoichiometric amount of fuel.
One particular type of engine technology capable of lean burning operation that has received much attention in recent years is known in the art by various names such as homogenous charge, or premixed charge compression ignition, or “HCCI”. In HCCI operation, fuel is typically delivered to an engine cylinder relatively early in an engine cycle, such that there is relatively more time available for mixing of the fuel and air prior to ignition and combustion. Once within the cylinder, the fuel and air mixture is compressed until autoignition commences. The relatively greater amount of time available for the fuel and air to mix tends to result in a more rapid heat release than in conventional engines. In conventional compression ignition engines, the rate of heat release is controlled in part by the rate of fuel injection, whereas in spark ignited engines, heat release is controlled in part by a finite turbulent flame propagation traversing the combustion space. HCCI operation has neither of these natural controls.
While homogeneous charge operation has shown great potential for NOx reduction, there is still room for improvement. In the context of compression ignition engines in particular, there is a limit as to how lean the fuel and air mixture may be, while still reliably autoigniting. Cylinder pressures sufficient to induce autoignition of lean charges can be impracticable with conventional hardware. Moreover, many HCCI engines are only able to operate across a portion of their theoretical load range, due at least in part to the relatively high pressure spikes which can result from the rapid, fairly uniform ignition of the fuel and air mixture throughout the cylinder. In particular, the relatively larger amounts of combusting fuel necessary to accommodate larger loads can simply create more pressure than the engine components can withstand.
A further challenge to HCCI engine designers relates to the difficulty in autoigniting a fuel and air mixture at a desired time. HCCI engines lack a natural control mechanism for ignition timing, such as the spark timing and fuel injection timing of spark ignited and conventional compression ignition engines, respectively. Moreover, ignition timing in HCCI tends to be sensitive to speed and load changes, combustion characteristics of previous engine cycles, and the specific fuel formulation and decomposition properties. As much of the potential of HCCI strategies for improved emissions quality relies upon igniting a lean mixture at a prescribed time such as at or near top dead center, the technology has yet to fulfill certain of its promises.
One attempt at improving lean burning engine operation is known from U.S. Pat. No. 6,595,181 to Najt et al. Najt utilizes an engine operating scheme wherein a pulse jet of reacting fuel mixture from a pre-chamber mixes with an ultra dilute premixed fuel-air charge in a main chamber. After the charge has partially combusted, rapidly expanding combustion gases ignite the remaining ultra dilute mixture by compression ignition. In other words, in Najt et al. there appears to be an initial flame from the pulse jet which thereafter ignites sufficient fuel to raise the in-cylinder pressure to a level sufficient for compression ignition. While Najt et al. provides one means that may have applications in certain systems, the design has various drawbacks. For instance, only a portion of the charge can achieve the advantages typical of HCCI operation. Furthermore, to the extent that any true HCCI operation is possible with Najt et al., at higher speeds and loads the engine must switch over to a conventional combustion regime.
The present disclosure is directed to overcoming one of more of the problems or shortcomings set forth above.